Creating units and adding them to unit group

i have a trigger that says:

event: Nerubian warrior dies

Conditions: -

Actions: Create 5 archers at archer spawn
Set 'archers'(variable) = units in archer spawn

and it does not add the units in archer spawn to the unit group, what is another way of doing this?

"Set 'archers'(variable) = units in archer spawn" does not work
use
"Pick every unit in archerspawn and add to (playergroup)"